Editor

TRANSMISSION LINE MANUAL Publication No. 268

Central Board of Irrigation and Power Malcha Marg, Chanakyapuri, New Delhi - 110 021

CBI&P Panel of Experts on Transmission Lines Chairman

'.J. Varma D I\n Ahh I\A,~Ii~

CENTRAL BOARD OF IRRIGATION AND POWER Esrablished 1\127

OBJECTIVES

To render expertise in the fields of water resources and energy; To promote research and professional excellence; To provide research linkages to Indian engineers, researchers and managers with their cOWlterparts in other

countries andintemational organisations; To establish database of technical and technological developments, and provide information services; Teclmological forecasting.

ACfIVlTIES

1. AdvancemtDt of Knowledge iUld Tecbnologital Forecasting t' ~ .. ," '." ..... , '~'~''':. ~

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TRANSMISSION LINE MANUAL

Editors C.V.J. Varma

P.K. Lal

Publication No. 268

CBI&P Panel of Experts on Transmission Lines

P.M. Ahluwalia Chainnan

CENTRAL BOARD OF IRRIGATION AND POWER Malcha Marg, Chanakyapuri, New Delhi 110 021

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STANDING PANEL OF EXPERTS ON TRANSMISSION LINES

Y.N. Rikh Ex-Chainnan, UPSEB

V.D. Anand Ex -Chief Engineer, CEA

M.L. Sachdeva Ex-Chief Engineer, CEA

Chief EngineerlDirector (Trans. Design), CEA

Chairman P.M. Ahluwalia

Ex-Member, CEA

Members

7. Executi ve Director/Chief Engineer (Trans. Design), UPSEB

8. Executive Director/Chief Engineer Transmission Designs, MPEB

9. Executi ve Director/Chief Engineer Transmission Design, GEB

10. Director Bureau of Indian Standards

Umesh Chandra AGM

D. Chowdhury DGM

11. Vice-President (Engineering)/General Manager Engineering, KECIL-RPG Transmission Power Grid Corpn. of India Ltd.

6. S.N. MandaI, Chief Design Engineer NTPC/K. Mohan Das, Addl. Chief Design Engineer, NTPC

12. Vice-President (Technical) EMC

Convenor P.K. Lal

Director (E) Central Board of Irrigation and Power

Chaper 1 Introduction P.M. Ahluwalia VN. Rikh YD. Anand

Chapter 2 Tower Types and Shapes

Chapter 3 Tower Geometry M.L. Sachdeva H.S. Sehra

Chapter 4 Electrical Clearances M.L. Sachdeva

Chapter 5 Design Parameters

Chapter 6 Loadings Umesh Chandra D. Choudhury

Chapter 7 Design of Towers

Chapter 8 Testing of Towers S.D. Dand L. Khubchandani

AUTHORS

ASSOCIATED TRAN,SRAIL STRUCTURES lTD. (An Associate Co, of Gammon Group) GAMMON HOUSE, 2nd FLOOR,

VEER SAVARKAR MARG, PRABHADEVI, MUMBAI400 025.

TEL:5661400~1 ::xtn: 4086/4043

Chapter 9 : Tower Materials, Fabrication, Galvanisation, Inspection and Storage B.N. Pai

Chapter 10: Design of Foundation S.M. Takalkar D. Choudhury

Chapter 11: Construction of Transmission Lines M.V Subbarayudu

Each Chapter was finalised after Intense input by Shri P.M. Ahluwalia, Chairman of the Panel Covering Detailed Review, Modifications and Supplements followed by final Discussion and Acceptance by the Panel of Experts.

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OTHER CONTRIBUTORS 1. L.c. Jain, Ex-Member, CEA 2. H.S. Sehra, Ex-Director, CEA 3. Powergrid Corpn. of India

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The Central Board of Irrigation and Power brought out a manual on "Design of Transmission Line Towers" in 1977. The publication proved immensely popular and had to be reprinted twice because of its usefulness to utility engineers and manufacturers of transmission line towers.

There have been many important developments since publication of the manual in 1977. The central sector generating companies like National Thennal Power Corporation and National Hydro Power Corporation made considerable impact on the generation scenario as also on EHV systems required for evacuation of power from the generating stations and also on inter-connection between various states for integrated system operation within the region. The regional grids are all in operation now and Power Grid Corporation of India is engaged in the task of establishment of National Power Grid. There have been considerable technological developments in the field of transmission engineering and the HVDC transmission and 800 kV transmission are going to play an important role in the National Power Grid.

It was, therefore, felt necessary not only to revise the manual published earlier but also to make it a comprehensive one to include not only towers but also other aspects of transmission lines incorporating latest technological developments. Keeping this in view the Central Board of Irrigation and Power constituted a panel consisting of eminent transmission lines experts from all over the country in 1988-89 under the chairmanship of Shri P.M. Ahluwalia, Ex-Member, CEA, New Delhi to take up this important work. .

This Panel of Transmission Experts further set up in March 1992 a Steering Committee and also a Working Group to consider and make suitable recommendations on the implications of the proposed draft amendment to the Indian National Standard IS:802-1977 "Code for use of Structural Steel in Overhead Transmission Line Towers" issued in 1991 based on the 1987 draft on the report oflEC 826 of Intemational Electro-technical Commission. The outcome of efforts made by Steering Committee led to adoption of the probabilistic method of design as contained in "Guide for New Code of Transmission Line" published by CBIP in 1993. These recommendations were adopted in Part-I of IS-802 published in 1995.

The present document "Manual on Transmission Lines" is outcome of the ceaseless efforts made and voluminous work done by the Panel of Experts on Transmission Lines. The various chapters contained in the publication were authored by groups of eminent practising experts and were thoroughly discussed in the meeting of panel at the time of finalisation.

This publication will be immensely useful to Managers, Design and practising engineers of power utilities and Transmission Line Companies, Researchers, Testing Stations, Faculty Members and Students of Engineering Institutes in India and overseas.

The Central Board of Irrigation and power wishes to acknowledge its grateful thanks to the authors of the different chapters for their expert contribution. Special thanks are due to Shri P.M. Ahluwalia, Chairman of the panel for the tremendous input and direction given for finalising the manual. Shri V.D. Anand, Chief Engineer (Retd.), CEA took it upon himself to go through the final manuscript meticoulously and correcting the same. The Board is also thankful to the members of the Committee for their valuable contribution.

It is hoped that this publication will be well received by the engineering fraternity.

Vll

(C.V.J. VARMA) Member Secretary

Central Board of Irrigation and Power

Power projects are highly capital intensive. Transmission Line is the vehicle for optimum utilisation of power produced at power projects. Transmission Line suffers from limitless insurmountable handicaps - Funds, Environment, Ecology, Proximity of Objects. Forests, Right of Way, Changing Hostile Terrains, Uncertainties of Wind, Temperature, Snow and Lightning, and above all requirements of Reliability, Security and Safety. Overcoming all these adversities Transmission Line has to deliver to the consumer power at minimum cost and with maximum reliability.

Tower is the most critical component of Transmission Line. CBI&P published in 1977 "Manual on Transmission Line Towers". That document became very popular in India and Overseas with Power Utilities and Tower Manufacturers. It had to be reprinted two times in 1988-89, CBI&P set up a Panel of Experts on Transmission Lines to review the Document considering the latest technological developments.

In India, Towers were designed following Deterministic Method of Design as per Indian Standard, IS:802-1977 Code of Practice for Use of Structural Steel in Overhead Transmission Line Towers.

For almost a decade since 1980, CIGRE and IEC worked on the Probabilistic Method of Design for Overhead Lines, culminating in the publication of the Recommendatory Report IEC 826:1991, based on which CIGRE Working Group 22.06 sent a Questionnaire to various countries of the World, including India. The CBIP Panel of Experts on Transmission Lines examined the subject with speed and in depth through Steering Committee of top-most Transmission Experts. As a result India was one of the first countries in the world to adopt the Probabilistic Method of Design as contained in the sister Publication of CBI&P "Guide for New Code for Design of Transmission Lines in India" -1993. In accordance with the CBI&P Guide, Indian Standard IS:802" Code of Practice, for Use of Structural Steel in Overhead Line Towers" Part 1, Section 1 "Materials and Loads has been amended and published in 1995. Chapters 5 - Design Parameters -6 -Loadings; and 7 -"Design of Tower Members; of the Present Document deal with this subject. Other subjects dealt with in the Document are: Tower Types and Shapes - Chapter 2; Tower Geometry - Chapter 3; Electrical Clearances - Chapter 4; Testing" of Towers - Chapter 8; Tower Materials, Fabrication Galvanising Inspection and Storage - Chapter 9; Design of Foundations - Chapter 1 0; and Construction of Transmission Lines - Chapter 11.

Each one of the Chapters was authored by eminent practising Experts incorporating latest technological advancements and practices and reviewed in depth by the members of the Panel of Experts on Transmission Lines before adoption. Special attention was given towards simplicity, clarity and completeness to make each chapter self-contained in all respects giving practical examples of calculation to facilitate practical application without hinderance.

The Document has full acceptability as the Panel comprised managerial experts from Central Electricity Authority, Central Government Power Corporations, State Electricity Boards, Bureau of Indian Standards, Tower Testing Stations, Research Institutes and Transmission Line Manufacturing and Construction Companies.

The mass of technological work could be accomplished by the untiring labours of the authors, members of the Panel of Experts and their organisations who worked behind the scene, CBI&P Management, Shri C.V.J. Varma, Member Secretary and Shri P.K. Lal, Advisor and other officers and staff of the CBIP. They worked ceaselessly for almost 9 years. lowe limitless gratitude and personal thanks to them for their co-operation and kindness in this great technical endeavour.

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Power utilities, Transmission Line companies and their engineers located in the far-flung corners of India were always faced with the dearth of a single unified document on Design, Manufacture and Construction of Transmission Lines. This Manual will fill that void. It will be of great reference value to the Management and Practising engineers of Power Utilities and Transmission Line Companies, Researchers, Testing Stations, Faculty Members and students of engineering Institutes in India and Overseas.

P.M. AHLUWALIA Chairman

CBIP Panel of Experts on Transmission Lines

Foreword Preface

~: Introduction

1.1 Preamble 1.2 Develppment of Power Systems in India 1.3 Environmental and Ecological Awakening 1.4 Privatisation Wave - Impact on Transmission Systems in India 1.5 Philosophies in Design of Transmission lines 1.6 New Concepts in Transmission Line Design 1.7 Resume of Topics Covered In the Manual

'- 2. Tower Types and Shapes

2.1 Scope 2.2 Types of Towers

2.2.2 Self-Supporting Towers 2.2.3 Conventional Guyed Towers 2.2.4 Chainette Guyed Towers

2.3 Tower Shapes 2.4 Tower DeSignation

2.4.2 Suspension Towers 2.4.3 Tension Towers 2.4.4 . Transposition Towers 2.4.5 Special Towers

3. Tower Geometry

3.1 Scope 3.2 Tower Anatomy 3.3 Bracing System 3.4 Tower Extensions 3.5 Tower Outline 3.6 Tower Height 3.7 Tower Width 3.8 Cross-arm Spread 3.9 Typical lengths of Insulator Strings on

Transmission lines in India

4. Electrical Clearances

4.1 Introduction 4.2 Minimum Ground Clearance 4.3 Minimum Clearance above Rivers/lakes 4.4 Environmental Criteria for 800 kV line . 4.5 Air Clearances - General Consideration 4.6 Clearances and Swing Angles on Transmission lines in India 4.7 Conductor Metal Air Clearances

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4.8 Air Clearance - Analysis by CIGRE . 4.9 Phase-to-Phase Air Clearances 4.10 Clearance between Conductor & Groundwire 4.11 Effect of Span Length on Clearances 4.12 Clearances at Power Line CrOSSings 4.:13 Recommendation

ANNEXURES

Annexure I - Spacing between Conductors Annexure II - 'Swing Angle for 800 kV Anpara - Unnao Line for Insulator

I Strings and Jumper APPENDIX - Investigation Studies on Clearances and Swing Angles for

Indian Power System

5~ Design Parameters

5.0 Abstract 5.1 Transmission Voltage 5.2 Number of Circuits 5.3 Climatic CQnditions 5.4 Environmental and Ecological Consideration . 5.5 Conductor 5.6 Earth Wire 5.7 Insulator Strings 5.8 Span

6. Loadings

6.1 Introduction 6.2 Requirements of Loads on Transmission Lines 6.3 Nature of Loads 6.4 Loading Criteria 6.5 Transverse Loads (TR) - Reliability Condition

(Normal Condition) 6.6 Transverse Loads (TS) - Security Condition 6.7 Transverse Load (TM) during Construction

and Maintenance - Safety Condition 6.8 Vertical Loads (VR) - Reliability .Condition 6.9 Vertical Loads (VS) Security Condition . 6.10 Vertical Loads during Construction and Maintenance (VM) - Safety Condition 6.11 Longitudinal Loads (LR) -Reliability Condition 6.12 Longitudinal Loads (LS) - Security Condition 6.13 Longitudinal Loads during Construction and Maintenance (LM) Safety Condition _6.14 Loading Compinations under Reliability, Security and Safety Conditions 6.15 Anti-cascading Checks . 6.16 Brokel1wite Condition 6.17 Broken Limb Condition for 'V' Insulator String

7. Design of Tower Members

7.1 General 7.1.1 Technical Parameters

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7.2.2 Graphical Diagram Method 7.2.3. Analytical Method 7.2.4 Computer-Aided Analysis

7.2.4.1 Plane - Truss Method or, 2-Dimensional Analysis 7.2.4.2 Space - Truss Method, or 3-Dimensional Analysis

7.2.5 Comparison of Various Methods of Stress Analysis 7.2.6 Combination of Forces to determine Maximum Stress in each member

7.3 Member Selection 7.4 Selection of Material

7.4.1 Use of hot rolled angle steel sections 7.4.2 Minimum Flange Width 7.4.3 Minimum Thickness of Members 7.4.4 Grades of Steel

7.5 Slenderness Ratio Limitations (KUR) 7.6 Computation of UR for Different Bracing Systems 7.7 Permissible Stresses in Tower Members

7.7.1 Curve-1 to Curve-6 7.7.2 Reduction due to bIt Ratio

7.8 Selection of Members 7.8.1 . Selection of Members in Compression 7.8.2 Selection of Members in Tension 7.8.3 Redundant Members

7.9 Bolts and Nuts

Annexures I 1\ III IV V VI VII VIII IX X XI

XII XIII XIV XV

Conductor Details Earthwire Design Loads Graphical Diagram Method Analytical Method Computer Aided Analysis Input for 3D Analysis Output Giving Summary of Critical Stresses Chemical Composition and Mechanical Properties of Mild Steel Chemical Composition and Mechanical Properties of High Tensile Steel Section List Equal Section Commonly Used for Towers & As Per IS:808

(~art - V) 1989 UR Consideration for Bracing System in a Transmission Tower Permissible Axial Stress in Compression Reference Table for Maximum Permissible Length of Redundant Members Dimensions for Hexagon Bolts for Steel Structures

8. Testing of Towers

8.1 Introduction 8.2 Testing Requirements 8.3 Description of a Tower Testing Station 8.4 Calibration 8.5 Assembly of Prototype Tower 8.6 Rigging Arrangements and Location of the Loadcells 8.7 Test Procedure 8.8 Testing of Prototype Tower 8.9 Special Requirements

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8.10 Acceptance of Test Results 8.11 Material Testing 8.12 Presentation of Test Results

9. Material, Fabrication, Galvani$ing, Inspection and Storage

9.1 Scope 9.2 " Material Quality Control 9 . .3 Specific Requirements of Fabrication 9.4 Operations in Fabrication 9.S Tolerances 9.6 Shop ,Erection/Proto-type Tower Assembly 9.7 Galvanising 9.8 Inspection 9.9 Packing and Storage

Annexures I II III

IV V VI VII

Chemical Composition and Mechanical Properties of Mild Steel Chemical Composition and Mechanical Properties of High Tensile Steel

" (a) Properties of Equal Angle Sections as per IS : 808 (Part V) - 1989 (b) Properties of Unequal Angle Sections as per I~ : 808 (Part V) - 1989 (c) Properties of Channel Sections Unit Weight of Plates Dimenf;ions of Hexagon Bolts for Steel Structures Ultimate Strength of Bolts Properties of Anchor Bolts. Metric Screw Threads as per IS : 4218 (Part-3)-1976 with ISO

Appendices Appendix I ; Quality Assurance Plan

I. Introduction II. Quality Objective" III. Quality Policy IV. Organisation of Quality Control Department V. "Quality Planning VI. Design and Drawings VII. Company Standards " VIII. Control on Inspection-EquipmentsIToolsiGauges IX. Material Management X. Incoming Material Inspection XI. Pre-production XII. In-Process Inspection XIII. Inspection and Testing of Finished (Galvanised) Material XIV. " Storage, Packaging and Handling

Enclosures - A Sampling Plan for Incoming Material a. Sections, Accessories and Bought out Items b. Sampling Plan for Physical Properties" of Bolts, Nuts and Spring Washers c. Sampling Plan for Galvanising"Test for Threaded" Fasteners " d, Formats for Inspection Report for Steel StackinglPreliminary-(QCD-I) e. Format for Report on Bend Test f. Format for Report on Testing of Physical Properties

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k. Format for Inspection Report for Accessories - (QCD-4) I. .' Format for Inspection Report for Steel Test Tower - (QCD-5)

B. Sampling Plan for In-process Material (a) Procedure (b) Format for Quantity Control Report (c) Format for Loading Report of Crates (d) Format for Inspection and Loading Report of Fabrication Shop (e) Format for Inspection and Loading Report of. Model Assembly (f) Format for Inspection and Loading Report of Model Shop (g) Format for Out-right Rejection Slip (h) Format for Rectifiable Rejection Slip (i) Format for Weekly Records of ShiftWise Acid Strengths G} Format for Galvanising Process Inspection Report (k) Format for Galvanising Inspection Report (I) Format for Testing Concentration of Prefluxing and Degreasing Solutions

Appendix II: List of Machines required for a well-equipped Tower - Fabricating Workshop Appendix III : Workshop Chart Appendix IV : Process Flow Chart for Fabrication of Tower

10. Design of Foundations

10.1 General 10.2 Types of Loads on Foundations 10.3 Basic Design Requirements 10.4 Soil Parameters 10.5 Soil Investigation 10.6 Types of Soil and Rock 10.7 Types of Foundations 10.8 Revetment on Foundation 10.9 Soil Resistances for Designing Foundation 10.10 Design Procedure for Foundation 10.11 Concrete Technology for Tower Foundation Designs 10.12 Pull-out Tests on Tower Foundation 10.13 Skin Friction Tests 10.14 Scale Down Models of Foundation 10.15 Tests'on Submerged Soils 10.16 Investigation of Foundation of Towers 10.17 Investigation of Foundation of a Tower Line in Service 10.18 Repairs of Foundations of a Tower Line in Service 10.19 Foundation Defects and their Repairs

Annexures

Annexure - I . Annexure - II Annexure - III Annexure - IV

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Typical Illustrations Tower Foundation Design Calculation

Illustration - I Illustration II Illustration - III Illustration IV Illustration V Illustration - VI Illustration VII Illustration - VIII Illustration 'IX Illustration - X

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11. Construction of Transmission Lines

11.1 Survey 11.2 Manpower, Tools and Plants and Transport Facilities 11.3 Environmental Consideration 11.4 Statutory Regulation for Crossing of Roads, Power Lines,

Telecommunication Lines, Railway Tracks, etc. 11.5 Surveying Methods 11.6 Foundations 11.7 Erection of Super Structure and Fixing of Tower Accessories 11.8 Earthing 11.9 Stringing of Conductors 11.10 Hot-Line Stringing of E.H.V. lines 11.11 Protection of Tower Footings 11.12 Testing and Commissioning 11.13 References

Annexures

Transmission Line Manual Chapter 1

Introduction

CONTENTS

Page 1.1 PREAMBLE

1.2 DEVELOPMENT OF POWER SYSTEMS IN INDIA 2 1.3 ENVIRONMENTAL AND ECOLOGICAL AWAKENING 2 1.4 PRIVATISATION WAVE - IMPACT ON TRANSMISSION SYSTEMS IN INDIA 2 1.5 PHILOSOPHIES IN DESIGN OF TRANSMISSION LINES 3 1.6 NEW CONCEPTS IN TRANSMISSION LINE DESIGN 3

i" 1.7 RESUME OF TOPICS COVERED IN THE MANUAL '3

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TRANSMISSION liNE MANUAL

INTRODUCfION

1.1 PREAMBLE 1.1.1 Electrical energy, being the most convenient and cleanest form of energy, is finding the maximum usage the world over for development and growth of economy and therefore generation, transmission and utilisation of the same in ever increasing quantities as economically as the latest technological advancements permit, are receiving great attention. The technical, environmental and economic considerations involved in siting and development of power generation projects required for meeting the demand for electrical energy are gradually resulting in longer transmission distances and introduction of higher and higher transmission voltages, and use of high voltage direct current transmission systems. Thus transmission systems with voltages of 800 kV ae and t 600 kV de are already in operation in some of the countries and those with 1000/1100 kV ac and 750 kV dc have also been introduced :n some countries. In India, 66 kV, 132/110kV, 2301220 kV, and 400 kVae. and 500 kV dc systems are already in service and 800 kV ac systems are in the process of implementation. All these systems owe there reliable performanee to a great extent to dependable transmission lines. Tower constitute a very vital component of transmission lines, as these performs the important functions of supporting the power conductors and overhead ground wires at the requisite distances above ground level and maintaining appropriate inter -conductor spacings within permissible limits under all operating conditions.

1.1.2 With increase in transmission voltage levels, the heights as well as weights of towers have also increased and so has their cost. The transmission line towers constitute about 28 to 42 percent of the cost a transmission line. Therefore optimisation of designs of towers can bring about significant economy in the cost of transmission lines .. It is therefore imperative that transmissionline towers are designed so as to make use of materials and workmanship most effectively and efficiently.

1.1.3 The weight of a tower required for any specific applications is influenced to a great extent by the selection of tower configuration, choice of steel structurals for tower numbers, typ.e of tower, types of connections etc. On the basis of experience and designing skill, a tower designer can produce tower designs conforming to the governing specifications and bring about optimum reduction in tower weight without sacrificing stability and reliability features of the fiQished tower which are very important for structural reliability of a transmission line. These depend not only on the designs of tower and its foundation but also on the type of tower, development of structural arrangement of tower numbers, detailing of connections, quality of steel structural, accuracy in fabrication, proper soil investigations, use of foundations according to soil conditions at sites of tower installation, accuracy and adequate care in tower erection and proper maintenance of the erected towers.

1.1.4 Depending on the manner in which the towers are supported these fall in the following two broad categories :.

1. Self supporting Towers

2. Guyed Towers This Manual covers all aspects of designs of self supporting ~owers and their foundations in a comprehensive manner.

~ ~ 1.~ DEVELOPMENT OF POWER SYSTEMS IN INDIA 1.2.1 In India, development of power over the years has been phenomenal. The installed generating. capacity has risen from a mere 2301 MW in 1950-51 to 85940 MW on 31st March, 1997. Matching with the installed generating capacity, transmission Systems have also grown. In 1950-51 there were only about 2700 Circuit KM of 132 kV lines and 7500 Circuit Km of 66/78 kV lines. These have grown to about liOO Circuit Km of 500 kV of HYDC lines, 32200 Circuit Km of 400 kV lines, 76400 Circuit Km of 220 kV lines, 97200 Circuit Km of 132 kV lines and 37700 Circuit Km of 66 kV lines (total 245,200 Circuit Kms). Strong interconnected transmission networks have been developed by each Electricity Board within the State boundaries. Regional Grids interconnecting State Transmission Grids have been built facilitating uninterrupted transfer of power within the region. National Grid at 800 kV and 400 kV is in the process of coming up spear-headed by Power Grid Corporation of India. Highlights of the power systems in India are given in Exhibits tl to 1.7. International comparisons with other countries are given in Exhibits 1.8 and 1.9.

1.3 ENVIRONMENTAL AND ECOWGICAL AWAKENING 1.3.1 Environmental and ecological considerations were not given so much importance in the past in the designs of transmission lines and their routing. However, availability of more sophisticated facilities has made it possible to investigate into the effects of electric and magnetic fields associated with transmission lines and understand and better appreciate the possible adverse effects of the above fields. In order to ensure that these fields least affect the way of life and ecology, the conductor configuration, tower shapes and transmission line corridors are so chosen that the magnitudes of radio interference (RI), television interference (TVI), audio noise (AN) and electrostatic fields radiated by the transmission lines are within safe limits and ecology is affected the least.

1.4 PRIVATISATION WAVE IMpACf ON TRANSMISSION SYSTEMS IN INDIA 1.4.1 Exhibit No.l.lO gives an idea of the sector wise utilisation of funds as well as the total funds allocated for Power from 1951 to 1992 and the outlay for the 8th Five year Plan period. It shows that against the norm of at least 50% of the total allocated funds being utilised for Transmission and Distribution, the average availability of funds for Transmission and Distribution over the years 1951 to 1997 has been 32% only. This has resulted in lopsided development of T&D systems leading to most of the chronic problems faced by the consumers.

1.4.2 Development of power systems being highly capital intensive but essential for overall growth of economy, induction of Private Sector in the development of generation as well as T&D systems is engaging the attention of the Govt. of India. Some headway has been made as regards generation projects. However, the same has yet to take place for the T&D sector. With privatisation coming through for this sector also, the transmission system will get impatus for faster development. .

1.4.3 Needbased funds for development of transmission and distribution system during the 9th Plan period are of the order of about Rs. 110 thousand crores. These are over and above the fUl}ds required for generation projects which are about Rs. 160,000 crores for the 9th Plan period. It may not be physically possible for the country to make available funds of this order in the Pu blic Sector. Privatisation of generation projects is already underway. Many IPPs h~ve sponsored power generation projects which are actually not coming up physically. The main bottle-neck is transmission and distribution. l!nless a Private Sector Company has the facility to make returns from the power project, their interest in actual execution will be limited. For privatisation in Power Sector to take momentum, it is imperative for privatisation to take place in transmission and distribution, not limiting to power generation only.

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1.5 PHIWSOPHIES IN DESIGN OF TRANSMISSION UNES Before IEC:826 - "Report on Loading and strength of overhead lines' came out in 1985, 1987 and 1991, the design of transmission lines in India as also in several other countries was made as p.er design philosophy based on deterministic concept of Loadings and strengths with specified factors of safety {or the different operating conditions. Consequent to consideration of the approach outlined in IEC " 826. design philosophy based on probablistic concept with provisions relevant to Indian experience has been finalised for Transmission Une design and the existing 15:802 (Part 1/5ection 1) - 1995 code of practice for use of structural steel in overhead line Towers has been recast accordingly .

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The new concepts in transmission line design philosophy include the followi 19 major changes in the design method:-

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(iii) (iv) (v)

(vi)

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Design based on limit load concept; Use of probablistic method of Design; Use of Reliability levels in transmission lines design; Use of Co-ordination in strength of line components; Use of six basic wind speeds converted to 10-minutes average speeds orresponding to 10-meter height over mean retarding surface as the basis for wind loads on transmission lines instead of three wind zones corresponding to 3D-meter height over mean retarding surface in use earlier; Consideration of the effects of terrain category and topography of transmission line corridors in the design wind speeds; and Carrying out anticascading checks on all angle towers

1. 7 RESUME OF TOPICS COVERED IN, THE MANUAL 1.7.1 The topics covered in chapters 2 to 11 of this Manual are briefly described below.

1.7.2 Chapter 2 - Towers types and shapes

1.7.2.1 This chapter describes fully the types of towers, tower shapes and designation of towers and brings out the essential differences between the various types of towers and the factors for preference of a particular type of tower to other types for some specific considerations.

1. 7.3Chapter 3 . Tower Geometry 1.7.3.1 'This chapter describes the various portions of towers and details the factors which determine tower height, tower width at various levels and the spread of cross-arms. It also describes the various types of bracing systems, insulator stings, and gives details of their composition, typical details of 66 kV,' 132 kV, 220 kV and 400 kV insulator strings, values of angles of swing and corresponding electrical clearances for insulator strings and jumpers for transmission lines already in service in India. analytical calculations of electrical clearances on transmission lines etc.

1.7.4Cliapter 4 . Electrical Clearances 1. 7.4.1 This chapter covers the requirements regarding the minimum electrical clearances to be maintained at tower and at mid-span between live parts of transmi~on line and from live parts to tower members for the various types of over voltages to which transmission lines of different voltage levels are subjected in service. It also deals with the minimum ground clearances, effect of span length on clearances and

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the requirements regarding electrical clearances of power lines crossing over tele-communication .d' circuits. railway tracks rivers, lakes etc. J,

1. 7 .5Chapter 5 - Design Parameters 1.7.5.1 This chapter covers the electrical, climatic and geological environmental and ecological considerations which influence the designs of transmission lines. It deals with the effects of shielcUng of lee-ward conductors by the wind-ward conductors of bundle conductors, span terminologies and their-significance in tower design, conductor creap allowance etc.

1.7.6Chapter 6 - loadings 1. 7.6.1 This chapter defines the various types of loads. gives methods for their estimation for snow-free regions. deals with the Reliability Requirements - climatic loading under normal condition security requirements - Failure containment under broken wire condition, safety requirements loadings under construction and Maintenance and Anticascading Requirements

1.7.7 Chapter 7 - Design of tower-members 1.7.7.1 This chapter describes the methods of analysis of stresses in plane trusses and space frames, and deals with selection of grades and si~es of steel structurals for tower members, use, of high tensile steel and mild steel sections, slenderness ratio limits for members with calculated and uncalculated stresses. built-up members, permissible stresses in tower members and bolts, design of tower members and member connections.

1. 7 .8 Chapter 8 - Testing of Towers 1. 7 .B.1 This chapter deals with the purpose of testing of towers, describes a typical tower testing station, celebration of load cells, rigging arfangements, locations of load cells in the test set- up, testing procedure, sequence of test loading cases. acceptance of test results and testing of tower material.

1.7.9Chapter 9 - Tower Materials, Fabrication, Galvanization, Inspection & Storage . . . ,

1.7.9.1 This chapter deals with Material quality control, specific requirements of Fabrication covering preparation of structural assembly Drawings, shop Drawings and bill of materials, cutting means, operations in Fabrication such as straightening, cutting )i.e cropping, shearing, cutting, or saucing), binding, punching, drilling and marking tolerances, shop erection (horizontal or vertical), Method of Galvanising, Inspecti9n as per quality assurances plan, packaging of finished members and their storage. The chapter highlights the significance of planing as it has great bearing on optimum utjlisation of material and limiting the wastage. The chapter contains data on permissible Edge Security and Bolt Gauges, chemical and mechanical properties of Mild and High tensile steels, Properties of Equal! Unequal Angles, channels, Plates, Bolts/Nuts, and Anchor Bolts, it also contains a s'ample QAP, list of Tower Fabricating, Machinery; details of Galvanising Plant. and the tests conducted on fabricated members.

1. 7.10 Chapter 10 Design of F uundations 1.7.10.1 This chapters deals with design requirements for various types of foundations for 3df -supporting towers. It brings out the importance of soil investigations and testing. classificaLivn ui soiis and excavations types of foundations and their application areas, procedure for their dC$igr,s l~tC. The chapter contains the permissible values of soil bearing capacities. permissible stress vaillt ': (,II concrete, reinforcement bar details and procedure for testing of foundatioils. A!)plkation of Ih~

11

11 b ir

E '

1 ~el

109 J. ,

y'. : uf r ' ~ of k.( ,(, 1/ t of t 1

:.! 5 and L,e

Is is

dempnstrated by typical detailed calculations of designs for aifferent types of Foundations.! he cnapter describes methods for investigating foundations and carrying out their repairs during construction 'stage and on lines in service.

1.7.11 Chapter 11 Construction of Transmission Unes 1.7.11.1 This chapter covers all the stages from reconnaissance survey up to commissioning of lines. It deals with statutory regulations, line corridor selection from environmental angle, methods of tower erection, paying out of conductors under uncontrolled and controlled tension, final sagging, clamping in. spacer Ivibration damper/spacer damper instaJIation, jumpering, live line stringing of EHV line,s, protection of tower footings etc. It also covers the tests to be conducted before line energisation.

5

EXHIBIT 1.1 Plan Outlays for Power Sector

For Plan Generation- T&D

First Plan (195156) Z83 110 Sa:ond Plan (195&-61) 310 116 Fourth Plan (1968-74) 699 321 Fifth Pl~ (1975--80) 1725 722 Sixth Plan (l98Q..85) 13851 5413 Seventh Plan

(l98~90) 25087 9185, Eighth Plan (1992-97) 57291 22280

NinthPlan 1,40,000 1,10,000 '(1997-2002)

O+------y----~r-----~~--~-

(Rs. Crores) Total

393

426

1020

2447

19264

34272

79571

2.so.ooo

I I t r

~ ~

EXHIBIT 1.2 Installed Generating Capacity

Yl'ar Nuch>ar Hydro Thermal Tolal

1 ~F)!) S I 0 S5~ 1742 2Wl 1

EXHIBIT 1.3 Electricity Generation

Year Nuclear Hydro

1950 .. 51 0 2860 1960-61 0 7837 1970-71 1339 25248 1978-79 2770 52594 i584 .. 85 4075 53948 1989-90 4625 62116 1990-91 6140 . 71640 1991 .. 92 5530 72760

~

199293 6730 69870 1993-94 5400 70460 199495 5646 82511 1995-96 7923 72383 1996-97 9024 68618

"-'0000

.Q)OO)

3~

:i JOOOOO

~ ~ :: ~ DlOOO , ..

"" lnm

1000.

~

0

Year

Thermal

~!i98 !iIOO 29961 4 715~J 9883i 178697 186550 208740 224760 24819() 267891""

299470 317158

(LW~L \1l:) Total

5~58 lti937

Sf:i54X 1/j2S23 1568[)9 245438 264330 287030

301360 324050 3S6i'l54 :179776 394800

_Nudear 1III(~dro ElThemul

i.a \'. 11

}-i':/ (oJ)

2:i1), l~ll

7 . { ~ } l

21

EXHIHIT 1.4 It'ngth of Transmission Unes (CK\1)

Transmis!lion 1951151 lYtifl-61 1970-71 1980-tH 1.985-86 1991l-91 1992-93 19959h lY9ti97 VI) \t(ijll'

HVl.C (Ol) tV

1321111) kV

TOlal \1)139

11111

4tili)l)

83140

Z340

3BM

59nx

7952

47XI)~

1 tjJI)

;jh]4

X: 4 h.'

1f,67 1&57

&'1186

120214 162!:140 20H521 218447 2045226

lXOOO.-----------------------------------------------__ ~ __ ~

ItXXXlO

l~H 1960-61 t.m71 1~81 1985-86 1990-91 199'2-93 1995-96 1996-97

9

B HVDC &400 \IV , o ZJOIZlO k. \' I II IJUIIOII" iii "78166144 kV

EXHIBIT 1.5 All India power Requirement Past Trend

Year Energy Peak Load Requirement

(MkWh) (MW)

1988-89 206331 :B551 1989-90 228662 36327 1990-91 246722 38986 1991-92 259000 41674 1992-93 282739 43636 1993-94 324417 54707 1994-95 349346 58904 1995-96 376679 63490 199&-97 413490 63853

~r---------------------------------------------~

I~

10lXXl0

o 19'~'9 1I~90 199G-91 199192 199291 1~94 1994-93 199'.96 1996-97

Var

EXHIBIT 1.6 AJI"lndia Power Requirement Forecast for 9th, 10th, 11th Plan

Year

1997-98

1998-99

1999-00

2000-01

2001-02

2006-07

2011-12

Source: 15th Electric Power Survey uf India

Energy Requirement

(MkWh)

436258 469057 502254

535903 569650 781863 1058440

Yell'

11

Peak Load

(MW)

734~8 7R936 84466

90093

95757 130944 176647

D FMrO'R .,dre_nt Puillold

EXHIBIT 1.7 Revised Fund Requirement Generation 1&D

(Rs. Billion) Year Capacity

Addition (MW) Generation T&D Total

9798 6000 210 126 336 9H-99 6500 227 137 364 991)1) 7000 245 147 392 1)1)01 7750 271 163 434 011)2 8500 297 179 476 02-03 9250 324 194 518 03\)4 . 10000 350 210 560 041)5 11000 385 231 616 05-06 12125 424 255 679

Total 78125 2733 1642 4375

Source: The India Infrastructure Report Published by Ministry of Finance Govt of India

IOOT-----------------------------------------~l~

L\\\\\"IT&D ! I'll4l.I Gcucrab:o

-+- Ca c' Additioo

Year

""r

~a Ch g

'- .. ~

'>' .,

HUI

lnd Iud,

r q J' 1~ J~r'\a Kon Mex ~,,(\

. K.h P' '1li PrJ'Jl

Sri 1.; I

Swedl U.K.

U~ U~_4l Yu .. s -SOL .f'

EXHIBIT 1.8 International Comparison (If Installed Capacity and Generation

r , Billion) Installed Capacity Generation

(M\\) (GWH)

Counlr~' /Yrar 1960 1970 1980 1990 1960 1970 1980 19~1!

Argetina 3474 6091 11988 17128 10459 2172i 396i6 4700i Bangladesh 990 2520 2&53 7732 Brazil 4800 11233 33293 52892 22865 45460 139485 211324 Canada 23035 42825 81999 104140 114378 204723 377518 440317 China 240180 67000 98600 59400 115900 300620 618000 Egypt 1167 4357 3583 11738 2639 7591 16910 37100 Finland 2834 4312 10422 13220 8628 21185 38710 45i36 France 21851 36219 62711 103410 72118 146966 246415 393713

) Germany 28393 47540 82585 99750 118986 242605 368770 389000 ,

Greece 615 2488 5324 8508 2277 9820 22652 34126 Hungary 1465 2477 4842 6603 7617 14541 23876 27463 India 5580 16271 31247 75995 20123 61212 112820 264300 Indonesia 319 907 2786 11480 1400 2300 6981 29810 Iran 2 2197 5300 17554 6758 17150 53200 Iraq 350 680 1200 9000 852 2750 8000 28410 Italy 17686 30408 46824 56548 56240 117423 185741 190327 Japan 23657 68262 143698 194763 115498 359539 577521 757595

~ Korea (DPR) 3400 5500 9500 9139 16500 35000 53500 .",'11 , Mexico J048 7318 16985 29274 10813 28707 66954 114277 : . Additicr! Norway 6607 12910 20238 27195 31121 57606 84099 108836 Pakistan 656 2334 2518 9137 26 8727 15277 37999 Phi11ipines 765 5176 4632 6869 2731 8666 18032 ~5249 Poland 6316 13710' 24723 30703 29307 64532 121871 128201 Sri Lanka 94 281 422 1289 302 816 1668 3150 Sweden 15307 27416 34189 34740 60645 96695 139515 U.K. 36702 62060 73643 73059 136970 249016 284937 298496 USA 186534 360327 630111 775396 844188 1639771 2354384 2807058 USSR 66721 166150 266757 333100 292274 740926 1293,878 1652800 Yugoslavia 2402 6972 14030 16470 8928 26024 59435 83033 Sourl"t': Powt'r Ot'vrlopment in India 1995-96

13

EXHmrr 1.9 International Comparison of Electricity Prices

(Indian Paise) Cost per Kwh

Country Industrial Domestic SI T.

Portugal 397 591 Germany 339 647 Italy 316 528 Spain 268 582 OBCD 258 378 United Kingdom 227 406 Denmark 221 666 Luxembourg 221 384 Ireland 215 432 Netherlands 202 415 Belgium 197 5tH Greece 197 341 France. 184 490 India 211 93

Source: Report on Energy Prices & Taxes lst Quarter 1995

700

600

SIX)

It

-1400 11.0 I 1 300

200

100

0 1 >. .. ; e i ~ i M

It ~ 110

E t ~ 'a i ~ j .B ] ~ I "II g 2 oJ :I

CAuIdry

EXHIBIT 1.10 Sectorwise Utilisation of Funds for Power ~tl

(Figurt's Rs. crorf's) SI. Period Total Funds Sector wise Utilis.ation No. utilised

for Power Generation Transmission & Others Distribution

Amount % Amount % Amount %'

1. 1st F.Y. Plan (195156) 260 105 40 132 51 23 ~ 2 . 2nd F.Y. Plan (1956-61) 460 250 54 115 . 25 95 21

. "

3. 3rd F.Y. Plan (1961-66) 1252 777 62 301 24 174 14 4. Annual F.Y. Plan (1966-69) 1223 676 55 291 24 256 21 5. 4th F.Y. Plan (1969-74) 2931 1505 51 768 26 658 23 6. 5th F.Y. Plan (197479) 7541 4467 59 2016 27 1058 14 7. Annual Plan (1979-80) 2473 1429 58 720 29 324 13 8. 6th F.Y. Plan (1980-85) 18913 12116 64 4706 25 2091 11 9. 7th F.Y. Plan (1985-90) 38169 24528 64 9847 26 3794 HI 10. Annual Plan (1990-91) 10470 7003 67 2375 23 1092 10 11. Annual Plan (199192) 13904 10373 75 2661 19 870 6 12. 8th F.Y. Plan (19297) Outlay 79730 49196 62 22432 28 8102 10

15

Transmission Line Manual Chapter 2

Tower Types and Shapes

CONTENTS

Page 2.1 Scope

2.2 Types of Towers

2.2.2 Self-S~pporting Towers 2.2.3 Conventional Guyed Tower 1 2.2.4 Chainette Guyed Tower 7 2.3 Tower Shapes 7 2.4 Tower Designation 7 2.4.2 Suspension Towers 8 2.4.3 Tension Towers 8 2.4.4 .Transposition Towers 8 2.4.5 Special Tower

Page

1

1

1

1

7

7

7

8

8

8

8

CHAPTER 2

TOWER TYPES AND SHAPES

2.1 SCOPE

2.1.1 The tower of various shapes had heen used in the past without considering detrimental influence on the environment. With conservation environmentalists attracting the highest attention and the public becoming more and more conscious of the detrimental effects of transmission line towers on the environment and occupation of land, transmission line tower designers have been endeavouring to develop towers with sllch shapes which blend with the environment. Other factors responsible for changes in shapes of towers are the need for the use of higher transmission voltages, limitation of right-of-way availability, audible noise level, radio and T.V. interference, electrostatic field aspects, etc. The types and shapes of Transmission Line Towers used in India and in other countries are discussed in this chapter.

2.2 TYPES OF TOWERS

2.2.1 The types of towers based on their constructional features, which are in use on the power transmission line are

~ven helow :

I. Self-Supporting Towers

2. Conventional Guyed Towers

~. Chainette Guyed Towers

These are discussed in the subsequent paragraphs.

112.2 SelfSupporting Towers

Self-supporting broad based/narrow based latticed steel towers are used in India and other countries. This type \oftower has been in use in India from the beginning of this century for EHV transmission lines. Self-supporting towers e covered under Indian Standard (IS : 802) and other' ational and International Standards. These are fabricated, sing tested quality mild steel structurals or a combination f tested quality mild steel and High tensile steel structurals onforming to IS:2062 and IS:8500 respectively. As H.T.

1

steel conforming to IS: 8500 is not readily available in the country, steel conforming to BS 4360 Gd 50B/ASTM A 572IJ1SNDE or any other InternationallNationai standards can be used. Some of the countries such as ; apan, USSR, Austria, Canada, France, etc., have explored use of other material such as steel formed angle sections, tubular sections, aluminium sections, etc., for fabrication of towers. In the case of heavy angle and long span crossing towers, some of the countries namely Russia, Norway, France, etc. are using single phase self-supporting towers. Self-supporting towers usually have square/rectangular base and four separate footings. HoweveN'wer voltage narrow-based towers having combined monoblock footings may be used depending upon overall economy. Self-supporting towers as compared to guyed towers have higher steel consumption. Self supporting to\Ve~~~sed}~r compactline design. Compact tower may comprise fabricated steel body, cage and groundwire peak, fitted with insulated cross-arms. Qmwa~tion ~s also achi~ye~ . .E1..~angement of phases, ~ll~ing V insul_ator strings, etc. Compact towers iUiVereduced dimensions and require sm3iier right-of-way and are suitable for use in congested areas and for upgrading the voltage of the existing Transmission Lines ~Iso.

Self-supporting towers are shown in Figures 1 & 2.

2.2.3 Conventional Guyed Tower

2.2.3.1 These towers comprise portal structures fabricated in "Y' and "V' shapes and have been use~ in some of the countries for EHV transmission lines upto 735 kV. The guys may be internal or external. The guyed tower including guy anchors occupy much larger land as compared to self-supporting towers and as such this type of construction finds application in long unoccupied, waste land, bush tracts in Canada, Sweden, Brazil, USSR etc.

2.2.3.2 Compact guyed towers are used on compact lines. The phases are arranged in such. a way that the phases are not interspersed by grounded metal parts of Tower. The phases can be placed in different configurations and are insulated from the supports. The conventional guyed towers

ZONTAUWAS" OWER

...!.

.,a:.

B-DEL T AlCA T HEAD TOWER

'.

PHASE-I

. ' .

. ~ .!.. ~ .,a.

SINGLE CIRCUIT TOWER

C-VERTlCAUBARREL TOWER

I'HASE-2 PHASE-J

E-TRIPLE POLE STRUCTURE

FIGURE I : SELFISUPPORTING TOWERS

...

.a ..

-' .

J)()UBLE CIRCUIT TOWER

N

~ ~ "" ...,

~ ~ "" "" $::l

5.

I NSUI,ATED

'FABRICATED TOWER BODY

COMPACT TOWER

MULTICIRCUIT TOWERS

FIGURE 2

4

...

and compact guyed towers are shown in Figure 3.

2.2.4 Chainette Guyed Tower

Chainette guyed tower is also known as cross rope suspension tower, and consists of two masts each of which is supported by two guys and a cross rope which is connected to the tops of two masts and supports the insulator 'strings and conductor bundles in horizontal formation ..

For angle towers, the practice is to use three separate narrow based masts each for carrying one set of hundle conductors or ~lse self-supporting towers. Each

. narrow based mast is supported with the help of two main guys. Typical chainette guyed towers for suspension and angle location are shown in Figure 4.

2.2.5 Guyed towers will be~overed in a separate I ~anual 2.3 TOWER SHAPES

,

Tower shapes in use are as follows:

(i) Verticallbarrel Type

(ii) Horizontal/Wasp Waist Type

(iii) Delta/Cat Head

(iv) H-Structure Type

In India, tower shapes at (i) and (ii) are used for single circuit line whereas tower shape at (i) has been used for double circuit and multi-circuit lines. In other countries al the above shapes have been used. Tower shape at (i) is structurally more stable and ideally suitable for multi-circuit lines. whereas tower shape at (ii) offer better performance from the consideration of audible noise, radio and television interference i.. electrostatic potential gradient at ground level and at the edge of the right-of-way. These towers shapes are shown in Figures 1 & 2.

2.4 TOWER DESIGNATION

2.4.1 Broadly, towers are designated as under:

(i) Suspension Tower

(ii) Tension Tower

DOUBLE TENSION i

SUSPENSION

:INSULATOR STRING

FIGURE 6 : ARRANGEMENT OF INSPAN TRANSPOSITION

(iii) Transposition Tower

(iv) Special Tower

2.4.2 SUspension Towers

These towers are used on the lines for straight run or for small angle of deviation upto 2 or 5. Conductor on sUspelisioh towers may be sUpported by means of I-Strings, VStrings, or a combination of I & V Strings.

2.4.3 Tension Towers

Tension towers also known as angle towers are used at locations where the angle of deviation exceeds that permissible on suspension towers and/or where the towers are subject to upliti loads. These towers are further classified as 2/SO-15, 15.30, 30600IDead end towers and are used according to the angle of deviation of line. One of the classes of angle towers .depending on the site conditions is illso designated as Section Tower. The section tower is introduced in the line after 15 suspension towers to avoid Cil

"

Transmission Line Manual Chapter 3

Tower Geometry

3.1 Scope

3.2 Tower Anatomy

3.3 Bracing System

3.4 Tower Extensions

3.5 Tower Outline

3.6 Tower Height

3.7 Tower Width

3.8 Cross-arm Spread

CONTENTS

3.9 Typical Lengths of Insulator String on Transmission Lines in India

Page

1

3

5

6 6

23

26

28

3.1

3.1.

3.2

3.2 .

3.2'0-.1

3.2.,)

3.2.3.1

3.2.'1

3.2.4.1

3

5 r )

S

3

5

~

v"at""" v

TOWER GEOMETRY

1 SCOPE

3.2

3.2.1

The Chapter describes anatomy of tower and factors involved in determining the outlines of the towers. The selection of an optimum outline together with right type of bracing system contribute to a large extent in developing an economical design of transmission line tower. The geometry of a tower has also a bearing on aesthetic values. The tower anatomy and tower outline are discussed below:

TOWER ANATOMY

A tower is constituted of the following components as shown in Figure-1

Peak Cross Arm Boom Cage Tower Body Body Extension Leg Extension Stub/Anchor Bolts and Base Plate Assembly

A brief description of each component of the tower is given as under:

3.2.2 Peak

3.2.2.1 It is the portion of tower above the top cross arm in case of vertical configuration tower and above the boom in case of horizontal configuration tower. The function of the peak is to support the groundwire in suspension clamp and tension clamp at suspension and angle tower locations re-spectively. The height of the peak depends upon specified angle of shield and mid span clearance.

3.2.3 Cage

3.2.3.1 The portion between peak and tower body in vertical configuration towers is called Cage. The cross-section of cage is generally square and it may be uniform or tapered throughout its height depending upon loads. It comprises tower legs interconnected by bracings are used in the panel of cage where cross-arms are connected to the cage or where slope changes for proper distribution of torsion.

3.2.4 Cross-Arm

3.2.4.1 The function of a cross-arm in case of vertical configuration tower is to support conductor/ground wire. The number of cross arms depend upon number of circuits, tower configuration and conduc-tor/groundwire arrangement. The cross-arm for ground wires consists of fabricated steel work and that for conductor may be insulated type or consist of fabricated steel work. The dimension of a cross-arm depends upon the line voltage, type and configuration of insulator string, minimum fram-ing angle from the requirement of mechanical stress distribution etc. At large angle of line deviation, rectangular/trapezc1idal cross-arm with pilot string on outer side are used to maintain live conductor to grounded metal clearance. The lower members of the cross -arm are called main members and the upper members as tie members/compression members depending upon direction of vertical loads.

2

3.~.t

't3.2.E Cross arm--

3 ... ~"T-_ Bracing

Tower body 3.3.1

Concrete level Body extension G. l:.:..,;::;;::::11\

Ground level

Single Circuit Tower Double Circuit Tower 3.3.2

3.3.2. Vertical/Barrel Type Towers

Boom level

3.3.3

Bracing 3.3 n 1

Concrete level

'Horizontal/Wasp Waist .. Type Tower

Figure 1: Tower Anatomy

3.3."

ge

Waist lev

I.e level -

3.2.5 Boom

3.2.5.1 It is generally a rectangular beam of uniform cross-section in the middle, but tapered in the end sections and form part of horizontal configuration towers (self supporting, guyed etc.) The boom is attached to the tower body and it supports power conductors.

3.2.6 Tower Body

3.2.6.1 Tower body is the main portion of the tower to connecting cagelboom to the tower foundation or body extension or leg extension. It comprises tower legs inter-connected by bracings and redun-dant members. It is generally square in shape. In another arrangement, a tower body comprises two columns connected on one of their ends to the foundations and on the other ends to the boom to which conductors are attached through the insulator strings.

3.3

3.3.1

BRACING SYSTEM

Peak, cage, tower body, body extension, leg extension, etc. comprises legs, bracings and redundants. The bracing and redundants are provided for inter-connecting the legs as also to afford desired slenderness ratio for economical tower design. The Framing Angle between bracings, main leg members and (both bracing and leg member) shall not be less than 15 Bracing patterns are single web system, double web or warren system, Pratt System, Portal System, Diamond Bracing system, and multiple bracing system. Each of the bracking system, shown in Figure 2, is described below.

3.3.2 Single Web System

3.3.2.1 It comprises a system either of diagonals and struts or of diagonals only. In diagonal and strut system, struts are designed in compression while diagonals in tension, whereas in a system with all diagonals the members are designed both for tension and compressive loads to permit reversal of the applied external shear. This system is particularly used for narrow base towers~ in cross-arm griders and for portal type towers. This system can be used with advantage for 66 kV single circuit line towers.

It is preferable to keep the four faces identical in case of 66 kv single circuit tower using single web system as it results in lighter leg member sizes. Single web system has little application for wide base HV and EHV towers.

3.3.3 Double Web or Warren System

3.3.3.1 This system is made up with diagonal cross-bracings. Shear is equally distributed between the two diagonals, one in compression and other in tension. Both diagonals are designed for tension and compressive loads in order to permit reversal of externally applied shears. The diagonal bracings are connected at their cross points. The tension diagonal gives an effective support to the compres-sion diagonal at the point of their connections, and reduces the unsupported length of bracings which results in lighter sizes of bracings members. This system is used for both large and small towers and can be economically adopted through out the cage and body of suspension and small angle towers and also in wide base large towers. In lower one or two panels in case of wide base towers, diamond or portal system of bracing is generally more suitable from the consideration of rigidity. These bracings result in better distribution of loads in legs and footings.

3.3.4 Pratt System

3.3.4.1 Shear is carried entirely by one of the diagonal members under tension. Other diagonal is assumed to be carrying no stress Struts, i.e.,horizontal member in compression are necessary at every panel

4

Strut ~,....

J I , I

(al (bl Single Web System

A_..I.-_A

(e) Portal System

(g) Multiple Bracing System (Lighter Tower)

Hip Bracing

Warren System

(h)

,,' i " , . , ._.+._. , I ' , ,

,./ ,

View 1-1

View 2-2

Multiple Bracing System (Heavier Tower)

Tower Geometry

,,----, ' I '\ I I I I

Pratt System

Diamond Bracing System

3.

-:I.:

3.:

... 4

;,.4

,

nactive l..nber

~

\ ....L..

lO provlae commul1Y 10 me oraclng syslem. Aovamage or mls SYSlem IS mal me sizes or olagonal members would be small because these are designed for high slenderness ratio in order to make them in tension. This type of bracings result in large deflection of tower under heavy loadings, because the tension members are slender in cross-section than compression members for similar loading. If such a tower is over-loaded, the in-active diagonal will fail incompression due to large deflection in the panel, although the active tension member can very well take the tension loads. This system of bracing impart torsional stresses in leg members of the square based tower and also result in unequal shears at the top of four stubs for the design.

3.3.5 Portal System (Shear Divided 50:50 between Diagonals KSystem)

3.3.5.1 The diagonals and horizontal members are designed for both tension and compression forces. The horizontal members are supported at mid-length by the diagonals, one half of the horizontal mem-bers is in compression and the other half in tension. The portal system is used for approximately the same size of panels as that for Pratt System of bracings in conjunction with warren system of bracings. It has been found advantageous to use the portal system for bottom panels, extensions and heavy river crossings towers when rigidity is a prime consideration. If hill side or comer exten-sions are anticipated, the portal panel is particularly attractive due to its versatility of application.

3.3.6 Diamond Bracing System

3.3.6.1 Somewhat similar to the Warren system, this bracing arrangement can also be derived from the Portal system by inverting every second panel. As for each of these systems, all diagonals are designed for tension and compression. Applicable to panel of approximately the same size as the pratt and portal systems, this arrangement has the advantage that the horizontal members carry no primary loads and are designed as redundant supports.

3.3.7 Multiple Bracing System

3.3.7.1 The EHV towers where the torsional loads are of high magnitude, the cage width is kept large to resist the torsional loads. Standard Warren system, if used, give longer unsupported lengths of legs and bracings which increases the weight of tower disproportionately, for such tower, multiple system of bracings is used. The advantage of this system in addition to reduction in forces in the bracings is that the unsupported lengths of leg members and bracings are reduced substantially thereby increasing their strength and reducing the member sizes. Although there is an incre.as.e in the number of bolts, fabrication and erection cost, yet the above system gives overall reduction in weight and cost of steel.

The bracings on the transverse and longitudinal faces may be staggered as reduction in tower weight is achieved by staggering the bracings. The system is preferable only for suspension and medium angle towers. In heavy angle and dead end towers, in order to have more rigidity, bracing on transfers and longitudinal faces should not be staggered.

3.4 TOWER EXTENSIONS

3.4.1 Body Extension

Body extension is used to increase the height of tower with a view to obtaining the required mini-mum ground clearance over road crossings, river crossings, ground obstacles etc. Body extensions upto 7.5 m height in steps of 2.5 m can be used and thus form a part of standard tower. For body extensions having greater heights say 25 m, the suitability of the standard tower is checked by reducing the span length and angle of deviation. Practice in the tower industry is also to specify negative body extension i.e. a portion of the tower body is truncated.

For lines transversing in hilly terrain, negative body extension can be used in tension towers from consideration of economy.

6 Tower Geometry

3.4.2 Leg Extensions

3.4.2.1 Leg extensions are used either with anyone leg or any pair of legs at locations ~here footings of the towers are at different levels. Leg extensions are generally used in hilly regions to reduce benching or cutting. The alignment of leg extension is done with the first section of a tower. Installation of leg extension calls for high degree of expertise in tower erection.

3.4.3 Stubs/Anchor Bolts and Base Plate Assembly

3.4.3.1 Stubs/anchor, bolts and base plate assembly connect the tower body/body extension including leg extension to the foundations. Cleats are provided with the stub to offer resistance against uprooting 0f the stub. A stub set consists of four members whereas the number of anchor bolts depends upon uplift and shear on the bolts.

3.5 TOWER OUTLINE

3.5.1 Tower Outline is fixed from the requirement of minimum ground clearance, terrain type, right of way limitation, electrical clearances etc. Tower outline is defined in terms of the following parameters:

3.5.1.1 Tower Heights

Minimum ground clearance Maximum sag including creep effect of conductor Length of suspension insulator string assembly Vertical spacing between power conductors Location of ground wire Angle of shield Minimum mid span clearance Tension insulator Drop

3.5.1.2 Tower Width

At Base or Ground level At Waist level At Cross-arm/Boom level

3.5.1.3 Cross Arm Spread

Type of insulator string assembly Suspension, I-string or V-string. Tension Pilot Swing angle Suspension String Assembly Conductor jumper Phase to phase horizontal spacing Each of the above parameters is discussed in the subsequent paragraphs

3.6 TOWER HEIGHT

3.6.1 Minimum Ground Clearance

'\

.;.1

; of the l'l"hing , f leg

I: .J leg rooting Il:! upon

way neters:

IUlations

3.6.2

laid down by Power Telecommunication vo-oralnallon vUlIlIlllll~~, ncyulaLlvlI~ 'U' " ...... _ ... _ Crossing on Railway Tracks-1987 laid down by Indian Railways and other applicable regulations laid down by different National Agencies like Indian Road Congress, Ministry of Surface Transports etc. The values of clearances required for lines of different voltage ratings are given in Chapter 4 of this manual.

Maximum Sag including Effect of Conductor Creep

3.6.2.1 The size and type of conductor (AAC, ACSR, AAAC. ACAR, AACSR), climatic conditions(wind,temp,snow)and span length determined the conductor sag. The maximum sag of a conductor occurs at maximum temperature and still wind condition. The maximum sag is consid-ered in fixing the height of a line support. In snowy region, the maximum sag may occur at 0 and nil wind for ice coated conductors.

3.6.2.2 Creep in a conductor is defined as permanent set in the conductor. It is a continuous process and takes place throughout its life. The rate of creep is higher initially but decreases with time since in serVice. Creep compensation is provided by either of the following methods :-

(i) (ii) (iii) (iv)

Pretensioning of conductor before stringing Over tensioning of the conductor in the form of temperature correction By providing extra ground clearance By a combination of partly over tensioning of conductor and partly providing extra Ground clearance.

The procedure for determining sag and creep compensation in respect of conductor is dealt with in Chapter 5 of this manual.

3.6.3 Maximum Sag of Groundwire/Minimum Mid Span ClearanceS/Angie of Shield

The function of groundwire is to provide protection to the power conductors against direct lightning stroke and to conduct the lightning current to the nearest earthed point when contacted by a light-ning stroke. The above functions are performed by the ground wire (s) based on selection of angle of shield, mid span cfearance and coordination of groundwire sag with that of conductor. The material and size of groundwire (galvanized stranded steel, alumeweld, ACSR, ACAR, AAC, AACSR) depends upon the criteria for sag coordination and extent of mutual coupling. The effect of cre.ep in galvanised stranded steel groundwire being negligible is not taken in account while deciding the s.ag. The location of groundwire (s) determine the height of groundwire peak. Single groundwire has been used in India for transmission line towers upto 220 kV having verticallbarrel type configuration and two groundwires for horizontal/wasp waist type towers of all voltages and 400 kV verticallbarrel type towers.

The detailed procedure for coordination of groundwire sag. with that of power conductor and values of mid span clearances and angle of shield are dealt with in Chapters 4 and 5.

3.6.4 Length of Insulator String Assembly

3.6.4.1 The length of suspension insulator string in combination with minimum ground clearance and maxi-mum conductor sag determine the height of (i) lowest crossarm in case of verticallbarrel/Delta type suspension tower and (ii)boom in case of horizontal wasp waist type suspension tower whereas the length of suspension insulated string in conjunction with phase to grounded metal clearance deter-mines the spacing between cross- arms in case of verticallbarrel type tower. The length of an insulator string is a function of insulation -level (BIL and SIL), power frequency voltage (service voltage dynamic over voltage) and service conditions (Pollution, attitude humidity). The depth of the jumper is affected by phase to grounded metal clearance which its.elf is determined from BIL, SIL,

8 Tower Geometry

service voltage, short circuit level, altitude, humidity etc. For determining electrical clearances, the length of the suspension insulator string is defined as the distance between the centre line of con-ductor and the point of contact of ball hook/anchor shackle with the hanger/U-bolt whereas the length of tension insulator string is defined as the distance between the point of attachment of the string to the strain plate at cross arm upto the jumper take off point of tension clamp. The length of V string for the purpose of determining the height of tower is the vertical distance between the lower main member of cross arm and ,centre of lowest conductor. For preparing clearance diagram the nearest live part from the grounded metal has to be considered. The number and size of discs., length of single and double suspension and tension string for various system voltages are given in Chapter 4 of this manual.

Typical arrangements of Insulator Strings are shown in Figures as indicated below:

Figure 3 Figure 4 Figure 5

Figure 6

Figure 7 Figure 8

Figure 9 Figure 10 Figure 11 Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Typical Insulator String Arrangement for 220 kV AC Transmission Line Single Suspension Insulator String for 400 kV AC Transmission Lines Typical Arrangement of Single Suspension String for 400 kV Lines with Twin Bundled Conductor Typical Arrangement of Double Suspension String (For 400 kV Lines with Twin Bundled Conductor) Single Tension Insulator String for 400 kV Transmission Lines Typical Arrangement of Double Tension String for 400 kV Lines with Twin Bundled Conductor 400 kV AC "V" Suspension with AGS Clamp for Twin Moose 400 kV AC Quadruple V Suspension Set for ACSR Bersimis (35.1 6) Quadruple Deadend Assembly for 400 kV AC ACSR Bersimis 800 kV Single V-Suspension Insulator String for Quad "Moose" Bundle 300 KN x 2(31 pcs. per String) 800 kV Single V-Suspension Insulator String for Quad "Moose" Bundle 400 KN x 2(29 pcs. per String) 800 kV Double V-Suspension Insulator String for Quad "Moose" Bundle 300 KN x _ 2(31 pcs. per String) 500 kV DC "vn Suspension Insulator Strings for Four ACSR Bersimis (35.1 mm Dia) 500 kV DC Quadruple Tension Insulator String Four ACSR Bersimis

3.6.5 Vertical Spacing between Power Conductors/Minimum Vertical Phase to Phase Clearances/ Minimum Phase to Grounded Metal Clearances

3.6.5.1 The vertical spacing between power conductors and between power conductor and groundwire is controlled by mechanical considerations (galloping/clashing and electrical consideration) (phase to phase and phase to grounded metal clearance requirements. The minimum phase to phase and phase to grounded metal clearances are generally determined on the basis of lightning impulse levels for lines of voltages upto 300 kV.

For lines voltages as are 300 kV, the minimum phase to phase and phase to grounded metal clear-ance are based on switching impulse level. The minimum phase to grounded metal clearance is affected by power frequency. The dynamic over voltage/service, voltage, altitude, humidity and temperature also. The minimum phase to grounded metal clearance is ascertained from the light-ning impulse level for lines upto 300 kV and switching impulse level for lines voltages above 300 kV as also power frequency dynamic over voltagel service voltage considering altitude, humidity and temperature also. The minimum phase to phase and phase grounded metal clearances for different

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Typical Insulator String Arrangement

I I I I r I I :r""\ i I r"\

Double tension string

Figure 3. Typical Insulator String Arrangement for 220 kV AC Transmission Line

~DiSC insulators I

3300 min. 255

U-clevis Arcing

suspensIon

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Notes 1. Spring washers electro galvanised 2. Other ferrous parts not dip galvanised 1 12% tolerance on length of hardware" 4. Nominal spacing tolerance !(O.03xspacing+OJlmm for insulator discs only

Figure 5: Typical Arrangement of Single Suspension String for 400 kY Lines with Twin Bun died Conductor

12

B

A

Tower Geometry

-----------------------

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450 iii u

520 c: IV ....

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Note: 1. Spring washers electro 9alvilnised 2. Others ferrous parts not dip galvanised 3. ~2% tolerance on length of hardwilre 4. Nominal spacing tolerilnce !'O.03xspilcing+0.31 mm for insulator discs only

Figure. 6: Typical Arrangement of Double Suspension String \for 400 kV Lines with Twin Bundled Conductor!

100 ,100 Min. 35

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Figure 7 : Single Tension Insulator String for 400 kV AC Transmission Lines

2-2.5 mm thickness

mm minimum

5392 mm mOiximum" ---'-

.. -- . --- . --_. --/-ll...:.l-J..

5457 mm m

Anchor shackle Link strap

: H. Ii ball eye Arcin 9 horn Socke t clevis Yoke plate [rading ring Clevis eye Susp en sian clamp with retaining rod

Figur e

wi' T n In r" ~d ";In'' 'ct--

9. V Suspension with AGS Clamp for Twin Moose

_._. -" ---.-:::------~- bolt

1. Anchor shackle 2. link strap 3. H. H. ball eye 4. T. S. arcing horn 5. Socket clevis 6. Spacing yoke 7. Bottom guard ring 8. U clevis 9. Susp ension clamp 10. Armour rod

Figure :10. 400 kV A ( Quadruple V Suspension Set for ACSR Bersimis (35.1 ct

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1_ Nom. 5900 Min. 5745

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figure 11 Quadruple Oeadend Assembly for 400kV AC ACSR Bersimis

'1. Anchor shackle 2. Yoke plate 3. Anctllr shackle 4. Yoke plate 5. Arcing horn 6. Ball clevis 7. Socket clevis 8. Yoke plate 9. Anchor shackle 10. Yoke plate 11. (levis eye 12. Corona central ring 13. Sag adjusting device 14. Extension link 15. Dead end assembly

11650

:Q)- Tower Fitting 240

Figure 12: BOOkV

450

1800 max.

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r I Item Description Main material Reqd. Cat No. Ball Clevis ~t~'!t tension 2 '(3) ~~s~:~on I~ul Porcelain 29x2 @. Arcing horn Steel 2 (j) Arcing horn Steel 4 (]) Socket Clevis Ductile iron 2 G> Yoke Steel 1 @ Clevis eye Steel 4

rigure 'JL: oOOrinyle '{ -S\Jspt;.nSlun IoISU.dt0. S. "in~ fL. (JaL 'M .)S, B "d . 300KN X 2 (31 pcs./String)

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Item Description Hain material Reqd.

D 90 Clevis eye High tension "Steel 4 D Compression damp Aluminium 4 3) Horn holder Steel 2

1(4) Ball clevis High., tension sf~e 2 5) Suspension clamp Porclean 31x2 6) Arcing horn Steel 2 7) Arcing ring Steel 1 i) Corona shidd ring Aluminium ,

L Socket eye Ductile iron 2 Ir9) 90 Clevis Clevis Ductile iron 4

1~ Q.uadruple yoke Ductile 'ron 1 11) Yoke Steel 2 12) 90 Clevis Clevis Ductile iron 4 S~a adjustable pia e Steel 4

~4) Anchor shackle I ~t~~1 tension 4 (15) Adjustable pl.te Steel 2

I Suitable conductor size of clamp Hin. breaking strength of string excePt-compression clamp . Type of ballal"idsocket parts

Cat No.

,

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Note: 1. Genenll tolerance on

Ial Length of hardware components :!:53 mm lill Insulator string :!:190 mm

2. Air gaps between live parts and tower body shalt be as per clearance diagram

Figure 14: 800 kV Double Tension Insulator String for Quad 'Moose' Bundle 300KNx2 (31 pes./String)

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Figure 15: :!:500 kV DC IV' Suspension Insulator Strings for Four A CSR BERSIMIS. (35'.1 mm dial

130

,"",.

Nominal in central Dosition

725

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Figure 16: :!:500 kV DC Uuadruple'Tension Insulator String Four ACSR BERSIMIS

1. Anchor shackle 2. Yoke plate 3. Anchor shackle 4. Yoke plate 5. Ball clevis 6. Socket clevis 7. Spacing yoke 8. Anchor shackle 9. (orona ring 10. (levis eye 11. Spacing yoke 12. Adjus table plate 13. Extension link 14. Dead.end damp 15. Dead Elnd damp

I\) I\)

Qi ~ ...,

G) (I) o 3 (I) ~

lVYV~1 I~ ylv~" "I rlyur~:s I qC1J C1IIU I qUJ. vvnerever elevauon omerence oelWeen twO aOJacem tower is considerable, the vertical clearances betWeen phases at the tension tower is determined by phase to phase switching/lightning impulse clearance between the highest point of the shielding ring/atoning horn of the tension insulator string of the lower phase and the lowest point of the jumper of the upper phase.

3.6.6 Tension Insulator Drop

3.6.6.1 The tension string/assumes position along the line of catenary of the conductor and therefore its inclination with respect to horizontal varies with change in sag. The Tension Insulator Drop is the vertical displacement of the jumper leg point w.r.t attachment point of tension string at strain plate. The drop is maximum under maximum sag condition and is lowest at minimum sag condition. While drawing the clearance diagram it is necessary to check the clearance of jumper for both minimum and maximum drop conditions of insulator string.

3.6.6.2 In case of considerable difference in the elevations of adjacent towers, the jumper leg and of insulator string of the tower at lower elevation may go up due to null point lying outside the span and the insulator drop maybe negative leading to insufficient live conductor to grounded metal clearance between the jumper and the cross-arm. Under such cases, the jumper may be modified to obtain the appropriate clearance.

3.7 TOWER WIDTH

3.7.1 The width of the tower is specified at base, waist and cross-arm/boom level.

3.7.2 Base Width

3.7.2.1 The spacing between the tower footings i.e. base width at concrete level is the distance from the centre of gravity of the corner leg angle to that of the adjacentcorner leg angle. The width depends upon the magnitude of the physical loads imposed upon the towers (calculated from the size, type of conductors and wind loads) and also depends upon the height of the application of external loads from ground level. Towers with larger base width result in low footing cost and lighter main leg members at the expense of longer bracing members. There is a particular base width which gives the best compromise and for which total cost of the tower and foundations is minimum. Through experience covering over a number of years, certain empirical relations have also been developed which are good guide in determining the base width. The base width of the tower is determined from the formula as given below:

B B M K

=

=

=

=

k~M Base width of tower at ground level in centimeters Overturning moment, in kg-m A constant

The value of K varies from 1.35 to 2.5 and 1.93 is an average value.

The determination of the correct value of the constant for suspension and angle towers becaus.e of such a wide range suggested, may lead to differing results. With a view to arriving at a simpler relationship, Figures relating to total weight of tower and their base widths are tabulated in Table 3.2 for typical towers of all voltage classes both single and double circuits. It is seen that the base width generally varies between 1/4 to 1/6 of the overall height of the tower upto concrete level- the values may be 1/6 for suspension tower, 1/5 for medium angle towers and 1/4 for heavy angle towers. Where the way leave is a problem, the design is optimized with the maximum permissible bas.e width.

24

b I

,h

~------~----~--~~- 0: l I~a~ r- or a .. -jset Figure17(a):Vertical Spacing Between Two Adjacent Cross-arms or Two

Power Conductors in Case of Suspension Tower

9, should be limited to oc for determini.nO.,minimum vertical spacing /Fora:

/)

y L. T.~~ __ -+----I

Figure 17Ibl: Vertical SpiKing Bet"'een two Adjacent Cross...:.arms . or two Power Conductors in Case of Tension Towers

oc > 9.

Vertical spacing = Y +b+h Y+b+h

Depth of jumper terminal point below cross-arm level

D = 1.10 x Maximum electrical clearance corresponding to Bil or Sil

h=

a=

a=

D Cos 93 + (x, +BtC) Cos oc or

(D+X2+B+C) Cos oc (a+St Sin 41/2+off set) tan oc

D Sin 93 +(x, +B+C) Sin oc or

(D+X2+B+C) Sinoc

Y' = Sag of the minimum span specified

b=D Cos 93 +(x, +B+C) Cos oc or Whichever is greater

D Cos 9. + (x2+B+C) Cos oc h = (a+St Sin ,/2+offset) tan oc

a = D Sin 93 + (x, +B+C) Sin oc or

a = D sin 9. +(x2+B+C) Sin oc

Sag of minimum span excluding twice length of tens.ion insulator string .

This value may -be worked out for maximum sag as well as minimum sag and a relevant value is adopted.

26 Tower Geometry

In medium and heavy angle towers, for the bracings to carry minimum possible loads, it is sug-gested that the base width and the slopes of the leg members may be adjusted in such a manner that the legs when extended may preferably meet at the line of action of the resultant loads. This reduces the forces in bracings to a large extent and a stronger and more stable tower emerges.

Typical slopes of bottom most leg member with vertical for various voltage rating tower are given in Table 3.1

Table 3.1 Typi.cal Slopes of Tower Legs for Various Voltages

Voltage Rating Type of Towers Slope of Leg

Upto 220 kV Suspension 4_9 angle 70-11

dead end 8-13

400 kV and above Suspension 8 -12 angle 10 - 17

dead end 11 - 15

3.7.3 Width at Waist Level

3.7.3.1 Width at the waist level is defined as the width at waist line in case of horizontal/wasp waist towers. For horizontal configuration, the width at:the waist level is found to vary from 1/1.5 to 1/2.5 of base width depending upon the slope of the 199.

3.7.4 Width of Cross-Arm Level

3.7.4.1 Width at cross-arm level is defined as the width of the tower at the level of lower cross- arm in case of barrel type tower. This width is mainly decided by torsion loading. The torsional stresses are evenly distributed on the four faces of the square configuration tower. The larger width reduces torsional forces transmitted to the bracings below that level and thus helps in reducing the forces in bracings of the tower body. The cage width is decided in a manner that the angle between lower main member and the tie member of the same cross-arm and that between bracings and belts is not less than 15 in line with the general structural engineering practices as an angle less than 15 may introduce bending stresses in the members.

3.8 CROSS-ARM SPREAD

3.8.1 The cross arm spread of a suspension and a tension tower is a function of Basic Impulse Level/ Switching Impulse Level and power frequency over voltage, configuration of insulator strings, angle of swings of suspension string in case of suspension tower and that of jumper in case of tension tower, phase to phase spacing etc. These parameters are described in Chapter 4 of the Manual.

3.8.2 Length of Cross-arm for Suspension Towers

3.8.2.1 Alternative-I: Insulator String-I Configuration

The length of the cross-arm is determined corresponding to nil swing and two swing an ales and the

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28 Tower Geometry

load (maximum) and vertical load and